Abstract: An insulating liner for a water closet tank that has a restricted top opening is disclosed. In one embodiment, the liner is generally rectangular in horizontal cross section so as to define two opposed sets of liner side walls, and one set of the liner side walls converges inwardly and upwardly from a bottom wall of the liner. The liner is formed of a flexible and resilient material so that the liner can be collapsed for insertion into the tank through the restricted tank top opening, and then can be released to resiliently return to its normal configuration where it bears against the interior of the tank side walls.